1. Field of the Invention
The present invention relates to a vehicle intercommunication apparatus for making short distance communication using ISM (Industry Science Medical) band.
2. Description of the Related Art
In the microfilm of Japanese Patent Application Laid-Open No. 62-155535, there is disclosed a vehicle intercommunication apparatus in which a base station is mounted on a vehicle and a headset configured with a speaker, a microphone, a portable radio terminal, and the like is provided on a helmet of an occupant in order to enable communication between occupants of a two-wheeler so that communication can be made via the base station between occupants on the same vehicle or occupants on different vehicles. A technique which employs Bluetooth as a wireless communication standard of the vehicle intercommunication is disclosed in Japanese Patent Application Laid-Open No. 2001-148657.
Since a radio wave propagation has rectilinearity, communication quality is largely influenced by a relative relationship between a size of shielding object present between radio terminals and a wavelength. In the communication of ISM (Industry Science Medical) band using a frequency of 2.5 GHz band in a region near to the upper limit of UHF band, not only a vehicle body but also an occupant's body strongly acts as a shielding object.
The Bluetooth which is a short distance wireless communication system utilizes the ISM band of 2.5 GHz, so that when this is employed for intercommunication or inter-vehicle communication, the communication quality thereof strongly depends on a relative position relationship between an occupant and a radio terminal. Therefore, when communication is made between a Bluetooth antenna provided on the vehicle and a Bluetooth antenna on the occupant, the communication quality is strongly influenced by a position of the antenna provided on the vehicle, a sitting position of the occupant who is a communication target, an accommodating position of the Bluetooth terminal on the occupant, and the like.
When a plurality of Bluetooth modules are provided in order to increase the number of radio terminals communicable with one base station, there is a possibility that interference between antennas of the respective Bluetooth modules occurs.
In the case where an antenna of the portable radio terminal is provided only at one portion of the rear of the helmet or the like, even when the occupant inclines his/her head to the side, the antenna is hidden behind his/her head viewed from the base station so that there is a possibility that the communication is shielded.